Objective To establish a rat model of hypertensive disorder complicating pregnancy (HDCP) and observe the effect of HDCP on thyroid morphology and function in preterm rats at various gestational ages. Methods Tenweek-old SD pregnant rats were randomly divided into control and HDCP groups. Rats in the HDCP group were subcutaneously injected with L-nitroarginine methyl ester (L-NAME; 250 mg/ (kg·d)). Rats in the control group were subcutaneously injected with the same volume of normal saline. Preterm rats were obtained by cesarean section at 18 ~ 21 days of gestation. Body weight and length of the preterm rats were recorded. The liver and thyroid gland were isolated, thyroid hormone levels were measured in a liver homogenate, and thyroid morphology and ultrastructure were observed. Results Compared with the control group, blood pressure and urinary proteins of rats were significantly increased in the HDCP group (P< 0. 05). The weight and length of premature rats in the HDCP group were significantly lower than those in the control group (P< 0. 01). Compared with the control group, the TSH level of premature rats was significantly increased in the HDCP group on days 19, 20, and 21 (P< 0. 01), and there was no significant difference in T3 and T4 levels. HE staining showed that the follicular cavity appeared at 18 days of gestation in the control group, whereas the follicular cavity appeared at 19 days of gestation in the HDCP group, and the number of follicular lumens was decreased significantly. Transmission electron microscopy showed that the rough endoplasmic reticulum in the HDCP group was significantly swollen and degranulated, which were accompanied by retention. Conclusions An HDCP rat model was established successfully. Compared with the control group, premature rats in the HDCP group had intrauterine growth restriction, hypothyroidism, and delayed thyroid histological development, which may be caused by endoplasmic reticulum dysfunction.